A fuel cell comprises an electrolyte layer and a pair of electrodes placed on either side of the electrolyte layer, and generates electricity through an electrochemical reaction between fuel gas such as hydrogen and alcohol and oxidizing gas such as oxygen and air, which are supplied to the corresponding electrodes, with the aid of a catalyst. Depending on the electrolytic material used for the electrolyte layer, the fuel cell may be called as the phosphoric acid type, solid polymer type or molten carbonate type.
In particular, the solid polymer electrolyte type fuel cell (SPFC) using an ion-exchange resin membrane for the electrolyte layer is considered to be highly promising because of the possibility of compact design, low operating temperature (100° C. or lower) and high efficiency, as opposed to the solid oxide type fuel cell (SOFC).
There are various geometric arrangements for fuel cells. A planar type comprises a pair of flow distribution plates provided with grooves for defining fuel gas and oxidizing gas passages, and a planar electrolyte layer interposed between the flow distribution plates. A tubular type comprises a tubular casing separated by partition walls into fine passages (so as to form a plurality of cells) or a tubular casing separated by electrolyte layers into fuel gas and oxidizer gas passages.
In a tubular fuel cell, the tubular casing is formed by extrusion as disclosed in Japanese patent laid open publication No. 10-189017 and Japanese patent laid open publication No. 10-40934. By using solid electrolyte, the electrolyte layer can also be formed by extrusion along with the tubular casing. Slurry containing the material for the gas diffusion electrodes and the catalyst is passed through the gas passages or the tubular casing is dipped in a bath containing such slurry so that the slurry be deposited on the surface of the solid electrolytic layer. The slurry is then dried or otherwise cured so as to form the gas diffusion electrodes.
The recent expansion of the applications for fuel cells has given rise to a need for more compact designs, but the extrusion process is not suited for producing compact tubular fuel cells. For instance, it is difficult to evenly deposit the material for the diffusion electrodes on the surface of the solid electrolyte layer by passing slurry through the narrow passages. Variations in the thickness of the diffusion electrodes are detrimental to an efficient power generation.
In a fuel cell, because of the consumption of the fuel gas, the flow rate of fuel gas progressively diminishes along the direction of the gas flow. Therefore, if the cross sectional area of the gas passages is uniform from the upstream end to the downstream end, the flow rate progressively diminishes, and an efficient power generation cannot be attained. It is conceivable to reduce the cross sectional area of each gas passage from the upstream end to the downstream end, but such an arrangement is quite impossible if the tubular casing is formed by extrusion.
In some applications, because of the special configuration of the space available for installing the fuel cell, it may be desirable to form an L-shaped or U-shaped fuel cell, and to curve the gas passages inside the casing. However, if the tubular casing is formed by extrusion, it is impossible to make the gas passages to curve in intermediate parts thereof.